geotiff: writer return-type drift (to_geotiff/write_geotiff_gpu return None, write_vrt returns str)

[brendancol]

All three documented writer entry points live next to each other in xrspatial.geotiff's public API, but their return types drift:

function	declared return
to_geotiff	None
write_geotiff_gpu	None
write_vrt	str (the path written)

write_vrt returning the path is intentional (callers may have passed an auto-generated name and want it back), but the drift breaks mypy consumers who handle the three writers uniformly, and makes the Sphinx docs surface inconsistent.

Fix: make all three return str (the path they wrote to). The change is additive for to_geotiff and write_geotiff_gpu since existing callers discarded the None return. write_vrt's existing str contract is preserved.

When the writer was passed a file-like (io.BytesIO) rather than a path, return its getattr(path, 'name', None) or repr as best-effort, or fall back to None and document it. The exact rule lands in the PR.

Scope:

• xrspatial/geotiff/_writers/eager.py: to_geotiff
• xrspatial/geotiff/_writers/gpu.py: write_geotiff_gpu
• xrspatial/geotiff/_writers/vrt.py: write_vrt (already returns str; covered by the regression test)
• New regression test under xrspatial/geotiff/tests/